Britain must remain "open for business" despite this week's strike by public sector workers, a Government minister has said.
Immigration Minister Damian Green refused to rule out using troops to help man border checkpoints in order to ensure ports and airports remained open throughout Wednesday's one-day action.
The authorities have warned of severe disruption at London's Heathrow Airport in particular as passport and immigration staff join the walkout by up to 2.6 million public sector workers protesting against changes to their pensions.
BAA, the airport operator, has appealed to airlines to bring in their aircraft half full amid fears of delays of up to 12 hours for passengers waiting to pass through immigration.
While Mr Green acknowledged that passengers were likely to face disruption, he said it was essential that Heathrow and other airports continued to operate during the strike.
"It is very important, as a trading nation, that we remain open for business," he told Sky News.
He said the Home Office was still working on contingency arrangements for Wednesday. Asked whether military personnel could be drafted in to help, he refused to rule out the possibility.
"We are taking contingency measures. You wouldn't expect me to discuss the detail," he said. "We are finding volunteers from all over Whitehall to come in and be trained so that they can be safe in manning the borders and keeping our borders safe and secure on Wednesday - which is the main priority - at the same time as allowing travellers and airlines to go about their normal business."
However, Labour said contingency arrangements for manning the borders should already be in place and it accused Home Secretary Theresa May of allowing herself to be distracted by the row over her relaxation of border controls.
"In other public services arrangements have been made to make sure essential services are available and agreements have been reached between management and unions," said shadow immigration minister Chris Bryant. "It is astonishing that the Home Office has not developed robust contingency plans in the way other vital services have done."